Phase and frequency variation response circuit



Feb.24,1942. f H. M. BACH 2,274,184"

PHASE AND FREQUENCY- VARIATION RESPONSE CIRCUIT Filed June l2, 1940 5 SheetsSheet l 1 25 a ...J C

PHASE a; FREQUENCY FIG. 3A FIG. 3

INVENTOR. HENRY M; EACH BY i4 ATTORNEY Feb. 24; 1942. H. M, EACH 2,274,184

PHASE AND FREQUENCY VARIATION RESPONSE CIRCUIT Filed June 12, 1940 5 Sheets-Shet 2 -o o--o oa a 0 47 R.F. LF. AUDIO AMPLIFIER M'XER AMPLIFIER DETECTOR AMPLIFIER :il

- INVENTOR. H'ENRY IV1. BASH ATTORNEY Feb. 24, 1942. H. M. BACH 2,274,184

PHASE AND FREQUENCY VARIATION RESPONSE CIRCUIT Filed Jun 12, 1940 5 Sheets-Sheet 3 I. F. AMPLIFIER AUDIO AMPLIFIER MIXER OSCILLATOR f,

MULTIPLIER POWER 0 AMPLIFIER MIXER CRYSTAL oscfz REACTOR TUBE DISCRIMINATOR 86 INVENTOR. HENRY M- EACH ATTORNEY.

Feb. 24, 1942. I 2,274,184

PHASE AND FREQUENCY VARIATION RESPONSE CIRCUIT Filed June 12, 1940 5 Sheets-Sheet 4 FIG. 8

MASTER OSCILLATOR .140 .10;

J02 J POWER 1 L AMPLlFIER A E DISCRIMINATOR INVENTOR.

FIG. IO HELNRY M. BACI-l ATTORNEY.

Feb. 24, 1942.

H. M. BACH PHASE AND FREQUENCY VARIATION RESPONSE CIRCUIT Filed June 12, 1940 5 Sheets-Sheet 5 INVENTOR.

HENRY I M. EACH ATTORNEY.

Patented Feb. 24, 1942 PHASE AND FREQUENCY VARIATION RESPONSE CIRCUIT Henry M Bach, Radio Patents New York Woodmere, N. Y., assignor to Corporation, a corporation of Application June 12, 1940, Serial No. 340,053

5 Claims.

Ihe present invention relates to electrical phase and frequency sensitive devices or response circuits of the type in which the magnitude and upon the departure of the phase or frequency of an alternating signal from some predetermined phase or frequency.

Among numerous applications involving the indication of and/or control depending on phase or frequency changes in electrical systems, devices or circuits of the above character are employed in automatic frequency control systems for radio transmitters and automatic tuning control (AFC) arrangements in radio receivers to as a result of and in proportion to a deviation of the transmitting frequency from its assigned value or a detuning of a receiving circuit, respectively, said potential or current serving to readjust the frequency or correct the tuning to the desired value.

Another use of phase or frequency sensitive devices or circuits is as a detector or demodulator for receiving phase or frequency modulated carrier signals to convert the received signals into a direct current varying in amplitude proportionately to the variation of sound signals or other intelligence imparted upon the carrier frequency in the form of phase or frequency modulation.

An object of the invention is the provision of a phase and/or frequency sensitive circuit which is highly eflicient and capable of responding to slight variations in phase or frequency of an impressed alternating signal.

Another object is to provide a phase or frequency discriminating circuit for use in frequency control arrangements ,for radio transmitters or in automatic tuning (AFC) systems in radio receivers capable of responding to slight deviations in phase or frequency of the oscillations produced by a transmitter or the tuning adjustment of a receiving circuit, respectively, to ensure a highly constant and stable carrier frequency being transmitted or an accurate tuning of a receiver to the frequency of an incomingsignal.

Another object is the provision of an automatlc frequency or tuning control system suited for use in a radio transmitter or receiver or an equivalent electric circuit characterized by'high operational stability and efiiciency in its performance over a prolonged operating period.

Another object is to provide a phase and/or potential or current depends frequency sensitive device or detector for use in 55 a phase or frequency modulated transmitter or receiver adapted to produce a pair of demodulated signal energies varying in opposite phase relation, each of said energies serving to control a different operating characteristic of the transmitter or receiving circuit.

The above and further objects and advantages of the invention will become more apparent from the following detailed description taken with refcrence to the. accompanying drawings forming part of this specification and wherein:

Figure 1 shows a basic phase and/or frequency variation response circuit embodying the principle of the invention,

Figure 1A is a graph illustrating the results obtained with a circuit according to Figure 1,

Figures IE 'to ID are vector diagrams explanatory of the function and operation of the circuit according to Figure 1,

Figures 2 and 2A are circuit diagrams and an explanatory graph, respectively, showing a modification of the basic circuit according to the invention shown in Figure 1,

Figures 3 and 3A represent a further diagram and an explanatory. graph, respectively, of still another modification of the basic circuit according to the invention,

Figure 4 illustrates, partly in block diagram form, a frequency modulation receiver embodying a detector or discriminator constructedin accordance with the invention, v

Figure 5 represents, partly in block diagram form, a superheter'odyne radio receiver embodying a frequency sensitive circuit as a discriminator for obtaining automatic frequency control (AFC),

Figure 6 shows partly in block diagram form, a modified frequency. modulation receiver embodying a frequency variation response circuit according to the invention serving both asa discriminator or frequency modulation detector and as a means for obtaining feedback to improve the performance or operational stability of the receiver, I

Figure 7 represents, partly in block diagram.

form, a radio transmitter embodying a frequency sensitive circuit according to the invention serving. as a discriminator for a frequency control system for stabilizing the transmitting frequency, I

Figure 8 shows a modified basic circuit according'to the invention to produce'output energy having an amplitude proportional to the phase anldlor frequency of an alternating inputvsigna suppressor grid Figure 9 shows a radio transmitting system embodying a frequency indicating device constructed in accordance with the invention,

Figure 10 shows another basic frequency variation response circuit designed in accordance with the invention,

Figures 11 and 12 are circuit diagrams showing further modifications of a frequency variation response circuit, embodying the principle of the invention, and

Figures 13 and 14 are another use of the invention for indicating or measuring capacity or inductance values.

Like reference characters identify like parts throughout the different views of the drawings.

With the above and further objects, as will appear hereafter, in view, in general the employment of an e-lectro-mechanical vibrating element such as a piezo-electric crystal as a means to develop an auxiliary voltage or potential from an alternating input signal potential in such a manner that said input potential and said auxiliary potential will be variably phase shifted so as to produce by combination or mutual intermodulation by the aid of an electron discharge stream, output energy having an amplitude varying proportionately to the relative phase or frequency departure of the input signal from the resonant frequency of said crystal element. The latter is excited by the input potential substantially electronically by capacitative coupling with a virtual cathode or concentrated electron space charge established within the tube. The advantages of the invention among others reside in a substantially increased operational stability and efficiency and improved performance of the phase and/ or frequency conversion compared with frequency variation response or discriminating circuits of the type at present known in automatic frequency control circuits and frequency modulation radio receivers.

Referring to Figure 1 there is shown a basic circuit according to the invention wherein customary elements such as current supply sources, etc., not forming a part of the invention have been omitted for simplicity of illustration. Item It) represents an electron discharge tube, in the example a tube of the 6L7 type known as a mixer tube, comprising a cathode H, a first control or signal input grid l2, a second control or injector grid l3 surrounded by a screen grid M, a suppressor grid 15 and an anode or plate IS. The

IS in the example shown is externally connected to the cathode ll.

Alternating input signals of varying phase and/or frequency are impressed by way of terminals ab upon the grid l2 and the cathode H or ground or any other potential reference point of the system. Item I1 is a biasing net" work comprising a resistor by-passed by a condenser in the cathode return lead of the tube to provide suitable grid bias potential preferably in such a mannerthat the tube will operate upon the straight line portion of its grid voltage-anode current characteristic. The screen grid I4 is by-passed to ground for alternating signal potential through a condenser 2! and is further connected to the positive pole of a suitable current source indicated by the sign through a resistor adapted to adjust the screen potential to its proper value. Numeral 22 indicates a measuring instrument inserted in the screen grid circuit used for plotting the operating characteristic of the tube as shown in Figure 1A. The

diagrams illustrating the invention involves second control grid I3 is connected to ground through a piezo-electric crystal element 23 shunted by a high ohmic resistor 24 which may be replaced by a choke coil and serves to provide a, direct current return for the grid l3 to the cathode. The plate I6 is by-passed to ground for alternating signals by a condenser 25 and is connected to the positive pole of a suitable space current source indicated by the sign through a load impedance such as a resistance 26 in series with a current measuring instrument 21. A further by-pass condenser 28 is connected between the terminal of resistor 26 remote from the plate 16 and ground. Output potential variations developed by the resistor 26 are applied to a. utilization circuit to be connected to output terminals c-d in any suitable manner such as through coupling condenser 30.

The following is a description of the operation and function of the circuit described. Electrons emitted by the cathode I I will be accelerated by the screen grid I4 and inpart attracted and collected by the plate It thereby forming a quiescent or steady current flow through the plate circuit of the tube. Part of the electrons after passing the openings or meshes of the screen grid l4 will become decelerated by thenegative potential on the control grid l3 thereby forming a concentrated electron space charge or virtual cathode adjacent to the grid 13. The intensity or charge density of this virtual cathode or space charge will vary in the rhythm of the fluctuations of the electron space current, that is at the frequency of the input signals applied to the grid l2. The fluctuating space charge will induce a corresponding potential upon the electrode l3 by electrostatic coupling,

be in phase quadrature with respect to the input potential impressed upon the grid l2. Due to the presence of the crystal 23 connected between the grid 13 and cathode II, the phase of the potential established on the grid l3 will be dependent upon the relative frequency departure of the impressed signals from the resonant or tuning frequency of the crystal 23. Thus, if the impressed frequency is equal to the resonant frequency. of the crystal 23 or a harmonic thereof1 the crystal will be substantially equivalent to an ohmic resistance resulting in a quadrature phase relation (angle a in Figure 1B) betweer the potentials el and e2 on the grids l2 and 13 respectively, due to the capacitative coupling 0: the grid l3 with the virtual cathode or spac1 charge fluctuations which latter are in phas with the input signal frequency. Whenever th input signal frequency deviates from the reso nant frequency of the crystal 23, the latter-wil be equivalent either to an inductive or capacita tive impedance, see Figures 1C and 1D, respec tively, and accordingly the phase of the potentii established on the grid l3 will increase. beyon or decrease below the normal quadrature pha:

relation both according to sign and in proportio to the departure of the impressed signal frr quency from the resonant frequency of th crystal 23.

As a, result, the electron space current passir from the cathode II to the anode l6 will 1 subjected to a double control by the grids and 13 in accordance with potentials of the 'san frequency but of varying relative phase, resul ing in the generation of a component of the on put or plate current having an amplitude var ing proportionately to the relative phase ang between the potentials on the grids l2 and that is, the induced potential will,

ground for the grid l3,

that is in turn in proportion to the frequency departure of the impressed input potential from v the resonant frequency of the crystal 23. This output component having an amplitude responsive to the frequency variations to be detected is caused to-develop a corresponding output potential-between terminals cd by the provision of a suitable coupling network. in, the example shown a suitably designed low-pass filter, comprising theseries resistor 26 and the pair of parallel condensers 25 and 28. As is understood, the filter in the output circuit should he designed so as to present high impedance to current component varying in the rhythm of or at the rate of the frequency departure of the impressed input potential from the resonant frequency'of the crystal 23.

Referring to Figure 1A, there is shown a curve representing the plate current variations from the steady or'quiescent state as a function of the phase or frequency of the impressed input potential. If the latter is equal to the resonant-frequency of the crystal 23 the current variation z' will be equal to zero and will increase anddecrease, respectively, substantially linearly over a certain range as the phase and/or frequency of .the input signal deviates in either direction from the resonant frequency of the crystal 23. At the same time it was found that the screen grid current as measured by the instrument 22 undergoes a similar variation in dependence upon the phase or frequency of the input signal but in a direction opposite to the plate current variations. This is shown by the curve isg in Figure 1A.

The resistor 24' forming a return path to should advantageously have a high value on the order of to 5 megohms, this value, however, not being critical. The suppressor grid |5 maydbe omitted without afiecting the operation of the circuit. Alternatively, if a tube with the suppressor grid such as of 6L7 type as shown is used, the suppressor grid may also be tied to the plate I6 or the screen grid M. It will be further understood that in place of'the input grid l2, any other control element known per se may be provided to initially control the electron discharge current in the rhythm of the input signal frequency.

Referring to Figure 2, there is shown a .basic circuit according to the invention substantially similar to Figure 1 but including a small variable condenser 3| shunted across the crystal 23. In this case the input alternating potential impressed between the cathode H and grid I2 is advantageously of a fixed frequency such as the resonant frequency of the crystal 23. By adjusting. the condenser 3| the crystal may be varied within certain limits resulting ina relative frequency departure between the impressed potential. and' the crystal frequency, or in other words, a. conversion or translation of the capacityvariations of the connumerous other industrial the output the effective frequency of a nal frequency, ably in the form nected thereto ,having an Referring to Figure 3 there is shown a further basic circuit differing substantially from Figures 1 and 2 by the interchange of the function of the control grids l2 and I3. The electron tube shown in this embodiment .is ing a third (oscillating) grid l2 which may be either left idle or directly connected to the screen grid l4. As will be understood, any other electron discharge tube may be employed for the purpose of the invention comprising the following basic elements: a source of electrons or cathode, a first control element such as a control grid for initially controlling the electron discharge stream in accordance with an input siga second control element preferof a control grid having conthe piezo-electric crystal, means such as a positively biased screen grid interposed between both said control elements to produce a concentrated electron space charge or virtual cathode adjacent to the second control element and a plate or output electrode, whereby the crystal is excited from the input potential purely electronically by capacitative coupling with said space charge or virtual cathode to effect a double control of the electron current conveyed to the anode in the rhythm of the sameinput potential but at varying relative phase in proportion to the frequency deviation to be detected or translated in the output circuit'of the tube. According to this modification of Figure 3, the input potential is impressed upon the outer grid |3 remote from the cathode and the piezoelectric crystal 23 is connected to the inner grid |2 adjacent to the cathode II. There is furthermore shown a choke coil. 24 shunted across the crystal 23 to act as a D. C. return to ground in place of the resistor 24 provided in the previous illustrations. It has been found that this circuit operates in subtantially the same manner as the circuits with the exception that the phase of the plate current and screen current variations 1}. and isg is reversed as shown by the graph according to Figure 3A.

Referring to Figure 4, there is shown, partly in .block diagram form, a frequency modulation radio receiver embodying a discriminator or frequency variation responsive circuit of the type according to the invention. Phase or frequency modulated radio signals intercepted by a dipole antenna 3536 are amplified in a radio frequency amplifier 31 and applied to a mixer or frequency converter 38 wherein the high frequency signals are combined with signalsof different frequency generated by a local oscillator 39 to produce signals of a fixed int'ermediatefrequency. The latter are selectively and efficiently amplified in an intermediate frequency amplifier 40 and impressed upon a limiter ll of any known design to eliminate spurious amplitude modulation, in such a manner that pure phase or frequency modulated signals are impressed upon the detector or response circuit by way of a tuned circuit comprising a condenser 42 and inductance 43. The response circuit or frequency variation detector shown is substantially similar to that according to Figure 1 and it is understood that in this case the resonant frequency of the crystal 23 should be equal to the intermediate frequency of the receiver. As a result, there is obtained at the output of the frequency detector a potential amplitude varying proportionately to the phase or frequency deviation .of the impressed input signals, that is according to the of the 6A8 type havaccording to Figures 1 and 2 variation of sounds or other intelligence being the amplified output signals are applied to a suitable translating or reproducing device such as a loud speaker shown at 41. It has been found that the crystal 23 may be tuned both to the fundamental or a higher harmonic of the intermediate frequency of the receiver in both of which cases a low frequency or audio output signal is obtained. The crystal 23 may be shunted by a small variable condenser or may have a variable air gap to allow slight adjustment of its resonant frequency and alignment with the rest of the receiver. The higher the Q-value of the crystal, the steeper will be the slope of the characteristics as shown in Figures 1A, 2A and 3A and the smaller the minimum phase or frequency deviation that may be detected.

- Referring to Figure 5, there is in block diagram form, a further application of a circuit according to the invention in the form of a frequency detector or discriminator to obtain automatic frequency control in a radio receiver or equivalent electrical circuit. Radio signals intercepted by an antenna 50 are impressed by way of acoupling transformer 5| and a radio frequency amplifier 31 upon a frequency changer or mixer 38 to produce intermediate frequency signals of a predetermined fixed frequency which are amplified in the intermediate frequency amplifier 40. The local oscillator for the mixer stage in the example shown is of the regenerative type comprising an electron discharge tube 55, an oscillating or tank circuit 56 and a feed-back coil or tickler 51 arranged in the conventional manner. The amplified intermediate frequency signals are applied to a second detector 52 to produce audio signal variations which latter are amplified in an audio frequency amplifier 46 and applied to a loud speaker 41 or any other translating device in accordance with standard practice. Intermediate frequency potential is impressed upon the control grid l3 of a frequency discriminator tube of the type'according to the invention by way of coupling condenser 53 and grid leak resistance 54. The remainder of the discriminator shown is substantially similar to the circuit shown in Figure 3. to control the oscillating frequency of the ocal oscilator there is provided an electronic reactance tube 58 of any suitable type excited by a quadrature oscillating potential derived from the oscillating circuit 56 of the local oscillator by the aid of a phase shifting circuit comprising a resistance 60 and a condenser 61 in series, the latter serving to develop a quadrature potential which is applied to the control grid of the reactor The amplified quadrature current of the oscillating or tube 58. the latter is impressed upon tank circuit 56 through a condenser 62 in a manner well known in the art. The quadrature current injected by the tube 58 into the oscillating circuit 56 and as a result the effective oscillating frequency are controlled in accordance with the frequency responsive or discriminating potential generated in the output of the discriminator by connecting the plate 16 of the discriminator tube H) to the control grid of the reactor tube 58 through a bucking potential source 63 designed to counter-balance the voltage on the plate l6 when the oscillator is on its correct frequency. The

control circuit includes a low-pass filter comprising in the example shown a series resistor 64'. and

In order illustrated partly Ill -bodying a frequency grounding condenser 65 designed so as to pass fluctuation of a desired rate to compensate for carrier frequency fluctuations of a desired order, that is either slow or progressive fluctuations due to heat or temperature changes or periodic and rapid fluctuations due to fading or other causes. According to a specific modification, a pair of discriminating potentials may be derived from both the plate and screen grid circuits of the discriminator tube and each utilized to control a separate reactor tube associated with the oscillator with the time constants of the respective. networks 64, for both controls being different in such a manner that one control is effective in eliminating slow or progressive carrier frequency fluctuations due to temperature and other influences, while theother control is designed to reduce or eliminate rapid or periodic fluctuations of the carrier frequency. In a double control system of this typea phase inverter such as a vacuum tube should be provided in one of the control circuits .to ensure a control of the carrier frequency in a like sense.

Referring to Figure 6, there is shown a receiving system for frequency modulated signals emdiscriminator according .to the inventionserving to supply auxiliary output potential serving to frequency modulate the local oscillator to produce additional frequency modulation superimposed upon the frequency modulation of the incoming signal in the mixer stage and I. F. amplifier. The additional frequency modulation may be either in phase or in anti-phase to the modulation of the received signals to effect either a regeneration or degeneration of the signals in the output circuit in an effort to improve the performance of the receiver or to stabiliz its operation and remove non-linearity.

The receiving system shown from the antenna 35-36 to the loud speaker 41 is substantially similar to Figure 4. The local oscillator associated with the frequency changer or mixer 38 in the example shown has the form of a regenerative electron tube circuit comprising a triode 55, an oscillating tank circuit 56, and a feedback or tickler coil 51 all connected in the customary manner for generating sustained local oscillations. The latter are impressed upon the mixer 38 by capacitative or any other suitable coupling arrangement. In order to control the oscillating frequency in accordance with the invention, there is provided an electronic reactance tube 58 having its control grid excited in quadrature phase relation by the oscillating frequency by the aid of a phase shifting circuit comprising a resistor 68 and acondenser 6| in series, the amplified quadrature current being impressed upon or in jected into the tank circuit 56 through a condenser 82 to obtain variations of the effective oscillating frequency in accordance with the transconductance or amplification factor of the re- I actor tube 58 determined by, its control grid potential in substantially the same manner as described in connection with Figure 5. The oscillating frequency in the present example is controlled by a potential derived from the screen grid M of the discriminator or frequency sensitive circuit in such a manner as to produce additional frequency modulation of the local oscillations impressed upon the mixer 38 and combined with the incoming signal oscillations. To this end a suitable low-pass filter comprising series resistance 20 and by.-pass condensers 2i and 2| is connected in the screen grid circuit to develop both as a demodulator and designed to counter-balance the positive potential on the screen grid [4 and a choke coil 66 bypassed to ground by a condenser 65. I

It will be understood (that the output energy applied to the audio amplifier 46 may be derived from the screen grid circuit and the feedback control potential may be derived from the plate circuit of the discriminator tube It). It is possible in this manner to produce feedback which is either positive or negative to improve the signal strength or the stability and linearity of the receiver, respectively, as may be desired.

In Figure 7 there is shown, partly in block diagram form, a frequency modulated transmitting system embodying a frequency discriminator according to the invention for stabilizing and maintaining substantially constant the transmit; ting or carrier frequency. The system shown comprises a masteroscillator of the regenerative type comprising a vacuum tube 10, an oscillating tank circuit 'H and a feedback or tickler coil 13 connected in a conventional manner to generate sustained electrical oscillations the frequency of which i determined by the natural or resonant frequency of the circuit II. The frequency of the oscillations is controlled in accordance with a modulating signal such as the output of a microphone circuit or any other modulating source by the aid of an electronic reactor tube 80 having a control grid excited in quadrature by the oscillating frequency by the aid of a phase shifting network comprising a resistor 8| in series with a condenser 82 and a coupling condenser 83 effectively shunting the tube across the tank circuit H in a mannerwell known in frequency control systems of this type. The effective reactance reflected by the tube 80 upon the tank circuit II and in-turn the oscillating frequency are controlled by the signal potential applied to the grid or any other suitable control element of the reactor tube controlling its trans-conductance or amplification factor. In the example shown this is accomplished by the audio frequency transformer 85 connected to the output of a microphone circuit or any other modulating signal source. In this manner the frequency ii of the oscillator is modulated in accordance with the variations of the signal such as sound variations or the like to be transmitted. In addition to this control, there is provided a further control'according to the present improvement designed to stabilize-or maintain the normal or center frequency at a substantially constant value to ensure faithful frequency modulation substantially free from distortion and other defects accompanied by a variant or fluctuating carrier frequency. In the example shown the frequency modulated oscillations produced by the master oscillator 10 are impressed upon a frequency quadrupler stage 14 producing a frequency 4h which latter is impressed upon power amplifier 15 feeding an antenna 16 or any other utilization circuit. A fraction of the energy in the multiplier 14 is applied to a frequency converter or mixer 18 together with a fixed or reference frequency f2 supplied from a standard frequency source such as a piezo-electric crystal oscillator 11 to produce an output frequency which in the example shown will be equal to 4f1fz. The lat- Y ter is impressed upon the input grid 12 of a frequency response clrcuit designed according to the invention and being in the example illustrated of substantially the same type as that shown in Figure 1. As is understood, in this case the natural frequency of the crystal 23 i equal to 4,f1fz

' or a harmonic thereof resulting in an output potential developed by the impedance 26 whose amplitude will vary in either direction and in proportion to the departure of the generated carrier frequency from its assigned value. This frequency responsive output potential in the present example is impressed upon the control grid of the reactor tube by way of a low-pass network comprising a series resistance 86 and by-pass condenser 81 and a bucking potential source 84 designed to counter-balance the positive potential on the plate l6 when the carrier equals its assigned value. The network 86, 81 is designed to have a time constant so as to be responsive merely to comparatively'slow or proressive variations or fluctuations of the carrier frequency, that is to carrier frequency changes at a rate signal frequency applied through the transformer 85. In this manner carrier frequency fluctuation due to temperature, humidity or other changes affecting the transmitter may be substantially minimized or practically suppressed with the result of a substantially purefrequency modulated wave being applied to the antenna or any other utilization circuit.

Referring to Figure 8, there is shown a basic circuit of the invention embodying a modified output circuit to produce energy having an amplitude varying proportionately to the frequency deviation to be detected.

Referring to Figures 13 to 1D it is seen that in addition to the direct intermodulation or combination of the potentials e1 and ea resulting in a frequency responsive component in the output circuit of an electron tube, there is produced a further variation or component in the plate circuit at signal frequency determined by the sum of the potentials er and e2 as shown at ea in Figures 13 to 1D. This component in the case of resonance (Figure 1B) has a certain amplitude which will increase for higher frequencies (Figure 1C) and decrease for lower frequencies (Figure 1D) in that an additional component e2 will be generated in one case assisting (Figure 1C) and in the other case opposing (Figure 1D) the potential e1; In other words, the-resulting component wave in the output circuit at signal frequency will be amplitude modulated in accordance with the initial frequency modulation and potential of the thus obtained amplitude modulated signal wave modification of Figure 8 by the aid of a high frequency choke coil in the plate circuit and impressed upon a diode rectifier 9'! by way of a coupling coil 96 to demodulate the amplitude 'modulated waves into a modulating signal current in the ordinary manner. Item 98 represents the diode load resistor by-passed by a condenser 99. There is obtained in this manner by conversion of the frequency modulated waves into amplitude modulated waves followed by an ordinary detection, an output current or potential at the terminals cd having an amplitude varying in proportion to the relative frequency departure of the input signal from the resonant frequency of the crystal 23 in substantially the same manner as in the preceding exemplifications.

Referring to Figure 9, there is illustrated a further application 0f the invention serving a an frequency exactly below the variations of the modulating is developed according .to the ter. The latter 'tion v circuit.

proper portion of pervisory frequency control of aradio transmitcomprises a master oscillator in the form of an electron tube I having a piezoelectric crystal IOI and a tuned circuit associated v therewith in customary manner, a power amplifier I03 and an antenna I04 or equivalent utiliza- Oscillating potential is applied from the oscillator by the aid of a voltage divider upon the input grid I2 of a discriminator tube I0 designed in accordance with the invention. The output circuit of the tube in the example shown the plate circuit includes a current measuring instrument I05 preferably of the zero center type for visually indicating the deviation of the transmitting frequency from its assigned value, i. e. the fundamental or a harmonic frequency of the crystal 23. Under normal conditions, the instrument I05 will read zero and be deflected in either direction whenever the transmitting frequency increases beyond or decreases below its assigned value, respectively.

As is well known, the Q-value of a piezoelectric crystal is extremely high and accordingly a phase or frequency discriminator as proposed by the invention is highly sensitiv to the slightest variations in phase or frequency. In fact, it is possible to effect a frequency control employing a system of this type responsive to a single cycle and less deviation from the normal or assigned frequency. If on the other hand, a response toa greaterdeviation is a plurality of piezo-electric crystal elements may be employed to adapt the system according to desired, a network comprising the invention'for use as a discriminator or de-- I tector of frequency modulation involving a substantial frequency deviation. A simple arrangement of this type is shown in Figure 10 wherein the crystal element 23 is shunted by two additional crystal elements I06 and I01 in series. The natural or resonant frequencies of the crystals are designed in such a manner as to result in a band-pass frequency response characteristic in place of a sharply tuned resonance curve obtained with a single element. In this manner a greater frequency deviation may be detected and translated with a great accuracy and operational stability compared with frequency sensitive devices or networks heretofore known and used in the art.

Figure 11 shows another modification of a system embodying theprinciples of the invention. In this modification, vacuum tube I0 contains cathode I I, a control grid I2, a positive screen or accelerating grid I4 and a collector IS. The electron stream flowing from the cathode II toward the screen grid I4 is modulated at. signal frequency by the input applied between points a and b. Item I! is a condenser resistance network in the cathode lead to operate the tube on the the grid potential-plate current characteristics. Most of the electrons will pass through the structure of grid I4 and will form a concentratedspace charge in front of the plate I6 biased negatively with respect to the cathode.

' bias resistor-condenser network I'I current flowing through it. Above and below the resonant frequency of the crystal there will be a lag or a lead of the current due to the properties of the crystal as has been described hereinbefore. Accordingly, the potential impressed upon grid I2 of a further vacuum tube or electronic mixer I0 will vary in phase according to the frequency departure of the incoming signal from the natural or resonant frequency of the crystal. The input voltage of relatively fixed phase is applied to the other control grid I3 of vacuum tube I0. Accordingly, in the output resistance 26 of vacuum tube I0 there will be developed a voltage which will vary in magnitude as a function of the frequency departure of the incoming signal from the resonant frequency of the crystal in a manner readily understood from the foregoing.

Vacuum tube line portion of its 'I0"is operated on the straight curve by the provision of the and operates in a known manner toyield an output varying in amplitude as a function of the phase angle between the signals impressed upon the grids I2 and I3.

Figure 12 shows a modification of Figure 11 wherein the input voltage and the phase shifted voltage are simultaneously applied to the control grid of the vacuum tube I0 arranged to operate on the bent portion of its characteristic so as to represent a square law detector. As a result thereof, an output signal will be developed by the load resistance 26 varying in amplitude as a function of the frequency departure of the input signal from the natural frequency of the crystal Any other known means of rectifying the combined input potential and the phase shifted potential may be used for the purpose of the invention such as for example a diode rectifier.

Figure 13 shows a system embodying the principles of the invention for measuring capacity or inductance values. There is shown an electron coupled oscillatorcomprising a four element electron tube I I4 having a cathode I I0, oscillation control grid I I I, positively biased oscillating or anode grid H3 and an output electrode or plate II2. An oscillating circuit comprising an induction coil I01 shunted by a variable condenser I08 is coupled with the cathode and the grids I I I and l I 3 to form a regenerative self -excited oscillator. condenser and grid leak resistance associated with the oscillating circuit in the conventional manner. The oscillations generated are transformed to a plate circuit of the tube by electron coupling of the plate I I2 with the electron discharge stream and-oscillating potential developed by the resistance coupling network comprising condenser H8 and resistors H6 and H9 are impressed upon the grid I2 and cathode I I of a frequency variation response or sensitive circuit of the type according to the invention. In operation, condenser I08 is tuned'very accurately so as to adjust the frequency of the oscillator to the resonant frequency of crystal 23 thusoperating the Items H5 and H5 are a grid coupling,

the appended claims.

is equal to the capacity of the unknown condenser. condensers of different capacity ranges connected in parallel and accordingly by changing the capacity of the most suitable one of these condensers, the condenser of unknown capacity may be very accurately measured. This method is very advantageous inasmuch as the indicating meter I05 cannot be injured by an unbalance of the circuit since the highest plate current variation as shown in Figure 1A will not be enough to drive the pointer of the-instrument off the scale. It is only necessary, therefore, to tune condenser I08 rapidly until the point of plate current variation is reached and then to carefully tune I08 to the center of the operating region.

Figure 14 shows another method of measuring capacities embodying the principles hereindisclosed. In this figure the frequency of the oscillator is fixed. Oscillations are generated in the triode section of tube I0 of the pentagrid converter type by means of the quartz crystal 'I21 connected between the grid I2 and cathode II and by the-tuned circuit comprising inductance I22 shunted by condenser I23 and connected to the anode grid I24. Circuit I22I23 is designed to resonate at the oscillating frequency of the quartz crystal I21. Hence the space current emitted from the cathode and accelerated through grid I4 is modulated at the frequency of r oscillation of the quartz crystal I21. Crystal 23 is nearly of the same frequency or a harmonic of the frequency of crystal I21. There is further shown a known. variable capacity I08 shunted across the quartz crystal 23. tuned so as to give a zero reading in the plate current meter I05, i. e. that crystal 23 is tuned exactly to the frequency of the. crystal I21, as has been described in conjunction with Figure 13. The unknown capacity is then shunted across terminals X and condenser I08 is decreased so as to return meter I05 to its zero reading. The change in capacity of I08 necessary to return the plate current to the'center of its operating region is the capacity of the unknown condenser.

It will be apparent from the foregoing that all the circuits shown and described herein will be equally responsive to both phase and frequency modulated Waves inasmuch as phase modulation merely constitutes a specific type of frequency r modulation. Accordingly, the term ffrequency modulation as used herein and the ensuing claims is to be interpreted broadly to include ordinary frequency modulation as well as phase modulation. I

It will be apparent from the foregoing that the invention is not limited to the specific details and arrangement of parts and circuits shown and disclosed herein for illustration but that the underlying novel principle and inventive thought are susceptible of numerous variations and modifications coming within the broader scope and spirit of the invention as defined in The specification ,and drawings are accordingly to be regarded in an illustrative rather than a limiting sense.

I claim:

1. In a frequency variation response circuit,

the combination with a source of alternating potential, of a piezo-electric crystal, the relative frequency of said alternating potential with re-' spect to a resonating frequency of said crystal being variable, an electron discharge tube having means comprising a cathode and a positively biased electrode for producing anaverage elec- Condenser I08 may comprise several- Capacity I08 is f? tronspace current of predetermined value, means including circuit connections from said source to said tube to cause fluctuations of said space current in accordance with said alternating potential, a control electrode adapted to vary said space current in accordance with an electric potential difference applied therebetweenand said cathode, means'for producing a concentrated electron space charge adjacent'to said control electrode, further means for connecting said piezo-electric crystal to said control electrode and cathode, an impedance element pervious to direct current arranged in shunt relation to said crystal, said impedance element being designed to offer high impedance to alternating current to cause the average space current to increase and decrease in proportion to the relative increase and decrease, respectively, of the frequency of said alternating potential with respect to the resonating frequency of said crystal, an output circuit for said tube, and means operatively associated with said output circuitv for developing energy having an amplitude varying proportionately to the average space current variations of,

said tube.

2. In a frequency variation response circuit,

the combination with a source of alternating potential, of a piezo-electric crystal, the relative frequency of said alternating potential with respect toa resonating frequency of said crystal being variable, an electron discharge tube having means comprising a cathode and a positively biased electrode for producing an average elec element, said resistance being designed to offer high impedance to alternating current .to cause the average space current to increase and decrease in proportion to the relative increase and decrease, respectively. of the frequency of said alternating potential with respect to the resonating frequency of said crystal, an output circuit for said tube, and means operatively associated with said output circuit for developing energy having an amplitude varying proportionately to said average spacecurrent variations.

3. In a frequency variation response circuit, the combination with a source of alternating potential of varying frequency, of a piezo-electric crystal, an electron discharge tube having a cathode and an anode, a first control grid near said cathode, a second control grid near said anode and an accelerating grid located between said first and second control grids, means for maintaining said accelerating grid at a steady positive potential with respect to said cathode to produce a concentrated electron space charge adjacent to said second control grid, means including circuit connections from said source to said first grid to cause fluctuations of the electron space current passing from said cathode to said anode in accordance with said. alternating potential, means for connecting said crystal to said second control grid and cathode, an impedance element pervious to direct current arranged in shunt relation to said crystal, said impedance element being designed to offer high the combination with a source of alternating.

potential of varying frequency, of a piezo-electric crystal, an. electron discharge tube having a cathode and an anode, a' first control grid near said cathode, a second control grid near said anode and an accelerating grid located between said first and second control grids, means for maintaining said accelerating grid at a steady positive potential with respect to the cathode to produce a concentrated electron space charge adjacent to said second control grid, means including circuit connections from said source to said first grid to cause fluctuations of the electron space current passing from said cathode to said anode in accordance with said alternating potential, means for connecting said crystal to said second control grid and cathode, an impedance element pervious to direct current arranged in shunt to said crystal, said impedance element being designed to offer high impedance to alternating current to cause the average space current through said tube to increase and decrease in proportion to the relative increase and decrease, respectively, of the frequency of said alternating potential with respect to a resonating frequency of said crystal, an output circuit connected to said accelerating grid,and means operatively associated with said output circuit adapted to develop energy having an amplitude varying proportionately to said averag space current variations.

5. In a frequency variation response circuit, the combination with a source of alternating potential, of a piezo-electric crystal, the relative frequency of said alternating potential with respect to a resonating frequency of said crystal being variable, an electron discharge tube provided with a cathode, a first control grid, a second control grid and an accelerating grid located between said first and second control grids, means for maintaining said accelerating grid at a positive potential with respect to said cathode to produce a concentrated electron space charge adjacent to said second control grid, means including circuit connections from said source to said first control grid to cause fluctuations of the electron space current emitted from said cathode in accordance with said alternating potential, means for connecting said crystal to said second control grid and cathode, an impedance element pervious to direct current arranged in shunt relation to said crystal, said impedance element being designed to offer high impedance to alternating current to cause the average space current through said tube to increase and decrease in proportion to the relative increase and decrease, respectively, of the frequency of said a1- ternating potential with respect to the resonating frequency of said crystal, and an output circuit for said tube.

HENRY M. BACH. 

